The present invention relates to a speaker whose voice coil bobbin is supported by a plurality of dampers.
FIG. 5 is a cross sectional view showing a conventional speaker whose voice coil bobbin is supported by a plurality of dampers. In the speaker 1 shown in FIG. 5, a voice coil bobbin 3 is coaxially disposed within a casing 2. The voice coil bobbin is supported at the fore end (the upper end in the drawing) and at the mid position by a couple of dampers 4A and 4B such that the voice coil bobbin 3 is vibratory in the axial direction. Those dampers are located between the voice coil bobbin and the casing 2.
A voice coil 5 is put on the rear end (the lower end in the drawing) of the voice coil bobbin 3. A disc-like plate 6 is made of iron and has a center hole.
The plate 6 is coaxially disposed around the voice coil 5. A slight gap is present between the plate and the circumferential outer surface of the voice coil 5. The plate 6 is fastened to the casing 2.
A magnet 7 having a center hole is bonded to the rear side surface (the lower side surface in the drawing) of the disc-like plate 6 in a state that the magnet 7 and the disc-like plate 6 are coaxially disposed. A disc-like yoke 8 made of iron is bonded to the rear side surface of the magnet 7. A center pole 8A is integrally formed at the central part of the front side surface (the upper side in the drawing) of the disc-like yoke 8. The center pole 8A is inserted into the voice coil bobbin 3 from its rear end, and located therewithin in a state that the former is slidable relative to the latter.
This conventional speaker is arranged such that the voice coil bobbin 3 is supported by use of the two dampers 4A and 4B so as to hold a linearity of a vibration of the voice coil bobbin 3 when it axially vibrates by current of an acoustic reproducing signal flowing through the voice coil 5 in a magnetic field, which is developed, by the magnet 7, among the disc-like plate 6, disc-like yoke 8 and center pole 8A.
The conventional speaker 1 fails to perfectly hold the linearity of the vibration of the voice coil bobbin 3, however. The reason for this is as follows: The dampers 4A and 4B are located closer to the fore end of the voice coil bobbin 3 with respect to the magnetic circuit formed by the voice coil 5, disc-like plate 6 and center pole 8A. Therefore, the supporting positions of the voice coil bobbin 3 are disposed asymmetrically with respect to the magnetic circuit.
Therefore, when the voice coil bobbin 3 vibrates at a low resonance frequency f0 and its vibration amplitude increases, the voice coil bobbin 3 rolls or unintentionally moves. In this state, there is a danger that the voice coil 5 comes in contact with the disc-like plate 6. Particularly in the speaker for reproducing low frequencies, operation of the speaker for reproducing sounds at large output power levels is difficult.
In the structure of the speaker 1, the iron yoke 8 forming the magnetic circuit develops a magnetic field around the voice coil 5. Therefore, the size of the yoke 8 is large relative to the size of the whole speaker 1. Accordingly, the speaker 1 is heavy.
FIG. 6 is a cross sectional view showing another conventional speaker in which a voice coil bobbin is supported by a plurality of dampers. A speaker 10 of FIG. 6 is of the called inside magnet type, while the speaker 1 of FIG. 5 is of the called outside magnetic type in which the magnet is located outside the voice coil bobbin.
The speaker 10 is constructed such that a magnet 11 is located within a voice coil bobbin 12, and the rear end surface (the lower end surface in the drawing) of the magnet 11 is bonded to a yoke 13. The yoke 13 is formed with a disc-like bottom plate 13A and an outside center plate 13B located outside the magnet 11. The circumferential inner surface of the fore end of the outside center plate 13B is confronted with the circumferential outer surface of an inside center plate 14, which is bonded to the fore end surface (the upper end surface in the drawing) of the magnet 11 in a state that a voice coil 15 is interposed therebetween. In the thus constructed speaker, the voice coil develops a magnetic field.
The voice coil bobbin 12 is supported by a damper 17A and a damper 17B such that it is vibratory in the axial direction. The damper 17A is located between the fore end thereof (the upper end in the drawing) and a casing 16. The damper 17B is located between the rear end (the lower end in the drawing) and the outside center plate 13B.
In the speaker 10, the supporting positions of the dampers 17A and 17B are located symmetrically with respect to the voice coil 15. Therefore, a linearity of the vibration of the voice coil bobbin 12 is more reliably secured.
As seen from the figure, in the structure of the speaker 10, the damper 17B for supporting the rear end of the voice coil bobbin 12 is disposed inside the outside center plate 13B. Therefore, the diameter of the damper 17B is limited by the damper 17B.
For this reason, where the outside center plate 13B is small in diameter, the diameter of the damper 17B is correspondingly small. In this case, the vibration characteristic of the voice coil bobbin 12 is impaired. Where the diameter of the outside center plate 13B is excessively increased to increase the diameter of the damper 17B, the speaker 10 increases in size and weight.
The present invention has been made to solve the problems of the conventional speaker in which the voice coil bobbin is supported by a plurality of dampers. An object of the present invention is to provide a speaker in which the voice coil bobbin is supported by a plurality of dampers, which the speaker can produce sounds at high output power levels, and is reduced in size and weight.
A speaker defined in item 1 comprises: a first magnet disposed at a mid position of a voice coil bobbin while surrounding the circumferential outer surface of the voice coil bobbin mounted on a vibrating plate; a first voice coil wound on the circumferential outer surface of the voice coil bobbin at a position where the voice coil bobbin faces one end of the first magnet with a predetermined gap being interposed therebetween; a second voice coil wound on the circumferential outer surface of the voice coil bobbin at a position where the voice coil bobbin faces the other end of the first magnet with a predetermined gap being interposed therebetween; a center pole, disposed within the voice coil bobbin, for developing two magnetic fields in a state that the first and second voice coils are located between the one and other ends of the first magnet; and at least one couple of dampers for supporting the voice coil bobbin at position""s such that the voice coil bobbin is axially vibratory, the positions being located on both sides of and symmetrical with respect to those portions of the voice coil bobbin which are respectively wound with the first and second voice coils.
In the speaker, two magnetic fields are developed between the one end of the other end of the first magnet and the center pole by the first magnet in a state that the first and second voice coils are interposed therebetween.
When current is fed to the first and second voice coils located in those magnetic fields, the voice coil bobbin axially vibrates in accordance with an amplitude of the current.
The voice coil bobbin is supported at least one couple of dampers for supporting the voice coil bobbin at positions which are located on both sides of and symmetrical with respect to those portions of the voice coil bobbin which are respectively wound with the first and second voice coils. Therefore, even when the amplitude of the vibration of the voice coil bobbin is large, a linearity of the vibration is perfectly secured. There is no chance that the voice coil bobbin rolls or unintentionally moves.
A stable vibration of the voice coil bobbin is ensured even when the speaker is a woofer for reproducing low audio frequencies at high power levels. Accordingly, the speaker is capable of reproducing sounds at high fidelity.
The speaker of the invention does not need a large yoke extending to the inside and outside of the voice coil bobbin, which is indispensably used in the conventional speaker. Accordingly, the weight of the speaker is correspondingly reduced. Further, the damper supporting the rear end of the voice coil bobbin is not limited in size by the size of the yoke. Therefore, the size of the damper may be selected to be such a size as not to deteriorate the vibration characteristic of the voice coil bobbin. If the damper size is so selected, the weight of the speaker is not excessively increased.
Thus, the speaker of the item 1 is capable of reproducing sounds at high power levels, although its size is small. For this reason, this speaker is most suitable in particular for a sub-woofer carried on a car, which requires the specification of small size and weight.
A speaker defined in item 2 depends from the item 1. In this speaker, the center pole consists of a second magnet arranged to be opposite in polarity to the first magnet.
The center pole, disposed within the voice coil bobbin, for developing two magnetic fields in a state that the first and second voice coils are located between the one and other ends of the first magnet, is a magnet. Therefore, a large gap flux density is produced in each magnetic field.
Accordingly, the first magnet may be reduced in size by the increase of the gap flux. This leads to size reduction of the speaker. For the same size of the first magnet, the amplitude of the vibration of the voice coil bobbin is increased, and hence the acoustic power is increased.
A speaker defined in item 3 depends from the item 1. In the speaker, the center pole is made of magnetic material.
In the speaker of the item 3, two magnetic fields are formed, by the first magnet, in a state that the first and second voice coils are located between the one end and the other end of the first magnet and the center pole made of magnetic material. When current is fed to the first and second voice coils respectively located in those magnetic fields, the voice coil bobbin vibrates in the axial direction in accordance with an amplitude of the current fed.
A speaker defined in item 4 depends from the item 1. In the speaker, the first and second voice coils are connected in series or in parallel with each other.
In the speaker thus arranged, a current is fed to the first and second voice coils or currents as the result of equally dividing a current in value are fed to those voice coils, respectively. Therefore, the voice coil bobbin is stably vibrated.
To achieve the above object, in the speaker defined in item 5 which depends from the item 1, cotton wires electrically connecting the first and second voice coils are stitched into at least one of the dampers.
A speaker defined in item 5 depends from the item 1. In the speaker, cotton wires electrically connecting the first and second voice coils are stitched into at least one of the dampers. Therefore, there is no chance that tension of the connection cords does not affect the vibration of the voice coil bobbin in any way. Accordingly, a stable vibration of the voice coil bobbin is ensured.
A speaker defined in item 6 depends from the item 1. The speaker further comprises: at least one third magnet is disposed on the front or rear side of the first magnet; and at least one third voice coil wound on the circumferential outer surface of the voice coil bobbin at a location thereof facing the end of the third magnet which is opposite to the end thereof closer to the first magnet, the third voice coil facing the end of the third magnet which is opposite to the end thereof closer to the first magnet in a state that a predetermined gap is present therebetween.
In the speaker of the item 6, the third magnet is provided in addition to the magnets of the speaker of the item 1. Another magnetic field is formed between the third magnet and the center pole or the number of the magnetic fields is increased.
With provision of the third magnet, current is fed to the third voice coil, and the vibration of the voice coil bobbin is increased in amplitude. Accordingly, the acoustic power of the speaker is increased.